A pacemaker is an implantable medical device that recognizes various arrhythmias such as an abnormally slow heart rate (bradycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An ICD is an implantable device that additionally recognizes ventricular tachycardia (VT) or ventricular fibrillation (VF) and delivers electrical shocks to terminate these tachyarrhythmias. CRT is a procedure for use by biventricular pacemakers or biventricular ICDs to resynchronize the ventricles for use within heart failure patients.
AF is a type of atrial tachycardia detectable by pacemakers and ICDs wherein the atria of the heart beat irregularly. AF is a common comorbidity in patients with CRT devices (i.e. within biventricular pacemakers and ICDs equipped to deliver CRT to resynchronize the ventricles.) Approximately 35% of CRT patients have AF. In the majority of those patients who also have intact atrioventricular (AV) nodal conduction, impulses from the atria during AF are often conducted to the ventricles at rapid and/or irregular intervals leading to high ventricular rates, ventricular rate irregularity and/or loss of ventricular pacing. High ventricular rates, especially with concomitant rate irregularity, compromise hemodynamic performance, exacerbate left ventricular dysfunction (see, e.g., Melenovsky et al. “Functional Impact of Rate Irregularity in Patients with Heart Failure and Atrial Fibrillation Receiving Cardiac Resynchronization Therapy” European Heart Journal (2005) 26, 705-711) and may predispose the patient to acutely decompensated heart failure. Additionally, reduced biventricular pacing decreases the benefit of CRT.
One option for responding to an episode of AF is to deliver cardioversion shocks to terminate the episode, i.e. strong electrical shocks can be delivered to the atria in an attempt to revert the atria from fibrillation to a normal sinus rhythm. Although cardioversion can be effective in terminating an individual episode of AF, in many cases fibrillation eventually resumes, requiring another round of shocks. Repeated shocks are quite painful to the patient and can deplete battery resources of the implanted device. Because AF is not usually immediately life threatening, painful shocks for its treatment may be perceived by patients as worse than the disease itself and therefore not tolerated.
As some patients have hundreds of AF episodes annually, it is instead desirable to employ ventricular pacing techniques for pacing the heart during AF so as to mitigate the adverse effects of AF rather than terminating the episode using cardioversion shocks. In particular, it is desirable to pace the ventricles so as to stabilize the ventricular rate during AF. A particularly effective technique for controlling ventricular overdrive pacing during AF is dynamic ventricular overdrive (DVO), which is described in U.S. Pat. No. 7,308,306 to Park et al. DVO is also discussed in U.S. patent application Ser. No. 11/929,719, of Bornzin et al., filed on Oct. 30, 2007, entitled “Systems and Methods for Paired/Coupled Pacing and Dynamic Overdrive/Underdrive Pacing.” Briefly, with DVO, the ventricles are paced at an overdrive pacing rate selected to permit the detection of the least some intrinsic ventricular pulses, and then the overdrive pacing rate is dynamically adjusted based on the detected intrinsic ventricular pulses. In one example, an increase in the ventricular overdrive rate is performed only in response to detection of at least two intrinsic ventricular
beats within a predetermined search period. If at least two intrinsic ventricular beats are not detected within the search period, the overdrive pacing rate is decreased.
Another useful ventricular pacing technique is the floating base rate pacing technique set forth in U.S. Pat. No. 7,187,972 to Fain et al. Briefly, pacing techniques are described therein for maintaining a high target percentage of biventricular paced beats during AF, wherein the ventricles are paced in accordance with a floating mode-switch base rate (MSBR) during AF (or, more generally, during any “non-atrial tracking ventricular pacing mode” following a mode-switch from an “atrial tracking mode.”) Monitoring is performed to determine whether biventricular pacing in accordance with the MSBR satisfies a minimum acceptable pacing criterion (MAPC). The MSBR is increased and pacing is performed in accordance with the increased MSBR, when the MAPC is not satisfied. The MSBR is periodically decreased and biventricular pacing is performed in accordance with the decreased MSBR when the MAPC is satisfied. Herein, the technique of U.S. Pat. No. 7,187,972 and related techniques are referred to as Floating Base Rate (FBR) pacing techniques. [It may also be referred to as the Fain/Ostrow pacing technique.] FBR pacing can be used in heart failure patients receiving CRT, which seeks to resynchronize the ventricles while achieving a high percentage of paced ventricular beats.
Hence, various ventricular pacing therapies have been developed for controlling pacing of the ventricles during AF to mitigate the effects of AF, including procedures adapted for use with heart failure patients receiving CRT. As noted, some of these techniques seek to achieve a high percentage of paced ventricular beats. In some instances, this is performed without regard to the resulting mean ventricular rate, which, in at least some cases, can become elevated. Elevated ventricular rates, though, are seen by clinicians as disadvantageous, particularly within heart failure patients.
Accordingly, it would be desirable to provide techniques for controlling ventricular pacing during AF so as to reduce or minimize mean ventricular rates within at least some patients, and it is to this end that the invention is generally directed.